Put pumpkin in a saucepan. Pour vegetable stock over the pumpkin and add enough water so the pumpkin is completely covered; bring to a boil and cook until pumpkin is tender, about 20 minutes.
Drain pumpkin, reserving liquid, and transfer pumpkin to a large bowl. Mash pumpkin with a potato masher until smooth. Transfer mashed pumpkin to a saucepan.
Stir sour cream, butter, curry paste, coriander, and garlic into the pumpkin until the butter melts and the mixture is smooth; heat the mixture on low until warmed through, stirring small amounts of the reserved liquid into the mixture until you reach your desired consistency, at least 10 minutes.